1. Field of the Invention
The present invention relates to an extreme ultraviolet (EUV) light source apparatus to be used as a light source of exposure equipment.
2. Description of a Related Art
Recent years, as semiconductor processes become finer, photolithography has been making rapid progress to finer fabrication. In the next generation, micro-fabrication at 60 nm through 45 nm, further, micro-fabrication at 32 nm and beyond will be required. Accordingly, in order to fulfill the requirement for micro-fabrication at 32 nm and beyond, for example, exposure equipment is expected to be developed by combining an extreme ultraviolet (EUV) light source generating EUV light having a wavelength of about 13 nm and a reduced projection reflective system.
As the EUV light source, there are three kinds of light sources, which include an LPP (laser produced plasma) light source using plasma generated by applying a laser beam to a target, a DPP (discharge produced plasma) light source using plasma generated by discharge, and an SR (synchrotron radiation) light source using orbital radiation. Among them, the LPP light source has advantages that extremely high intensity close to black body radiation can be obtained because plasma density can be considerably made larger, that the light emission of only the necessary waveband can be performed by selecting the target material, and that an extremely large collection solid angle from 2π steradian to 4π steradian can be ensured because it is a point light source having substantially isotropic angle distribution and there is no structural member surrounding the light source such as electrodes. Therefore, the LPP light source is considered to be predominant as a light source for EUV lithography, which requires power of 100 W or more.
Here, a principle of generating the EUV light according to an LPP system will be described. By applying a laser beam to a target material supplied in a vacuum chamber, the target material is excited and turned into plasma. Various wavelength components including the EUV light are emitted from the plasma. Then, by using a EUV collector mirror reflecting selectively a desired wavelength component (for example, a component having the wavelength of 13.5 nm), the EUV light is reflected and collected, and outputted to an exposure unit.
In such an LPP type EUV light source apparatus, the influence of ions and neutral particles emitted from the plasma at the time of generating the plasma by applying the laser beam to the target particularly has been problematic. Especially, the influence of the ions and neutral particles emitted from the plasma upon the EUV collector mirror is problematic because the EUV collector mirror is located near the plasma. The scattered materials from the plasma including the ions and the neutral particles and remains of the target material are called debris.
Japanese Patent Application Publication JP-P2005-268358A discloses a method of removing deposited materials such as debris emitted from the plasma and adhered to the surface of the collector mirror, of which reflectance is degraded, by applying an ion beam to the collector mirror in a state where a vacuum is kept in a vacuum chamber, in order to reuse the collector mirror.
When high density plasma is generated by applying the laser beam to the target, ions are accelerated and emitted as fast ions from the high density plasma. The energy of the fast ions reaches up to ten and several keV. The fast ions collide with structural member including at least one of the collector mirror holder and the target supply nozzle provided in the vacuum chamber in order to generate the extreme ultraviolet light, an inner wall of the vacuum chamber, and so on. And particles emitted from the structural member caused by the collision adhere to the surface of the EUV collector mirror after moving in the vacuum chamber. In that case, when the EUV light transmittance of atoms constituting the adhered particles is low, the reflectance of the EUV collector mirror will be abruptly degraded. The structural member, such as the collector mirror holder, the target supply nozzle, or the inner wall of the vacuum chamber, is generally made of stainless steel in many cases. When the fast ions collide with the structural member made of stainless steel, particles including iron (Fe), nickel (Ni), chromium (Cr), etc. will be emitted. The particles move within the chamber and some of them adhere to the EUV collector mirror. Since atoms such as iron (Fe), nickel (Ni), and chromium (Cr) have a low transmittance for the EUV light, the reflectance of the EUV collector mirror is abruptly degraded.
The EUV collector mirror is required to have high surface flatness of about 0.2 nm (rms), for example, in order to maintain high reflectance, and therefore, the EUV collector mirror is very expensive. Accordingly, longer life of the EUV collector mirror is particularly desired in view of reduction in operation cost of the exposure equipment, reduction in maintenance time, and so on. Thus, the fast debris such as the ions and neutral particles emitted from the plasma collide with the structural member within the vacuum chamber, and thereby, the particles emitted from the structural member and including atoms having a low transmittance for the EUV light, move in the vacuum chamber, adhere to the EUV collector mirror, and reduce the reflectance of the EUV collector mirror. Such problems as mentioned above have been arisen, and resolution of the problems has become a subject.